If you have once good cockerel, one causing problems, and three others, I would definitely butcher the one that's causing problems. That will leave you with the good one and (for now) three extras.
On the whole I would agree with your MIL's advice. You may not be able to tell which one is best at this age, but I think you can recognize and remove some of the worst (the child-attacker) and the not-so-good (the one currently under discussion.)
This one is showing problematic behavior, and might injure the younger pullets, so for their safety I think he should be removed.